The present invention relates to multistage refrigeration systems and processes, and in particular to the use of a thermal reservoir in an intermediate refrigeration loop for storing thermal reservoir material in heat exchange relation with the refrigerant in that intermediate refrigeration loop.
In its simplest form, a refrigeration system provides a means for transferring heat away from an object or space to be cooled. Various schemes have been developed to accomplish this end. The heat transfer agents or media used in refrigeration systems known in the art include water, aqueous brines, alcohols, glycols, ammonia, hydrocarbons, ethers, and various halogen derivatives of these materials. While many of these materials provide effective heat transfer media under certain conditions, physical considerations eliminate many of them from use in various settings. Such considerations include adverse environmental impact, since many known heat transfer media have been implicated in the depletion of the ozone layer, and thus have been banned from use. Another factor is toxicity. For example, ammonia and many ethylene glycols have a level of toxicity that make them dangerous to handle and unsuitable for commercial food handling environments. Some heat transfer agents are flammable materials, such as some ethers and some hydrocarbons. The risk of flammability is particularly great where the heat transfer media is subjected to large positive pressures within the refrigeration cycle. Other heat transfer agents are disfavored because they are in a gaseous state at normal operating temperatures. Again, ammonia is an example of this type of refrigerant. Gaseous heat transfer media requires special high pressure equipment, such as pressure regulators and reinforced tubing, that are not required for refrigerants that remain in a liquid state through most or all of the refrigeration system's operating cycle. In addition, high pressure systems are prone to leakage. Some other heat transfer media are corrosive in nature, and thus not preferred. Many of the aqueous brines fall into this category, and thus require special handling provisions such as Teflon.TM.-lined conduits and interfaces, which add significantly to the overall cost of the refrigeration system. Furthermore, restrictions on the selection of materials usable with corrosive agents decreases the overall efficiency of these refrigeration systems.
A relatively new type of refrigeration system is known as a secondary loop refrigeration system. Such a system has shown significant advantages in terms of energy efficiency over conventional refrigeration systems, particularly in a supermarket environment. Secondary loop refrigeration systems are more compact in design, can be factory built, and are capable of operating with an extremely small charge of refrigerant. Furthermore, in secondary loop refrigeration systems, the vapor compression process of the refrigeration cycle is centralized, and can be operated from a remote location. Thus, the compressor in a secondary loop refrigeration system can be place on a rooftop, in a ventilated machine room, or in any other convenient location where it will not occupy valuable floor space or contribute to background noise, and where the effects of possible refrigerant leakage are minimized. Further, since the primary refrigerant loop running through the compressor is segregated from the secondary refrigerant loop used to cool the goods being refrigerated, the primary refrigerant loop may utilize ammonia or other high efficiency refrigerants that are unsuitable for use as direct refrigerants in many applications.
Sherwood, U.S. Pat. No. 5,819,549 (which is incorporated by reference herein), discloses secondary loop refrigeration systems. In one embodiment (Example 12 in the Sherwood '549 patent), a secondary loop refrigeration system is disclosed for use at stadiums and arenas. The refrigeration problem presented at such venues is quite different from that faced in the supermarket environment. At any event where a large number of people will gather for a few hours (and in particular, at sporting and entertainment events), there is a need to have mass quantities of cooled beverages on hand for immediate consumption by the thousands or tens of thousands of patrons at the event. For example, there will be about 63,500 spectators at a sold-out Minnesota Vikings home football game at the Hubert A. Humphrey Metrodome in Minneapolis, Minn. The spectators will typically consume 29,500 16-ounce cans of beer, as sold individually throughout the stadium by beer vendors, and 5,800 20-ounce plastic bottles of soda, as sold individually throughout the stadium by soda vendors. All of these sales take place within approximately a three to three and one-half hour period, and thus the operator of the stadium must necessarily have a suitable quantity of chilled beer cans and soda bottles on hand when the gates open.
This presents a rather unique challenge in terms of refrigeration. Of course, a traditional refrigeration chamber could be used to slowly cool the required quantity of beer and soda, if the venue had a cooler large enough. However, providing a cooler of that size would be impractical, for the type of usage incurred. Rather, a more appropriate solution is a "blast cooler" which will quickly chill the beer and soda (or other products which require quick chilling) for rapid distribution and consumption in such a stadium or arena setting. This reduces the footprint necessary for the cooling apparatus, and also reduces the time necessary for cooling. Another constraint placed on such venues are back-to-back events. For instance, in the Metrodome, it is possible to hold a Minnesota Golden Gophers college football game on a Saturday evening (ending about 10:00 p.m.) to be followed by a Minnesota Vikings professional football game on a Sunday afternoon (with the gates opening at about 10:30 a.m.).
While it may work to rapidly cool cans and bottles of beverages quickly in bulk, the secondary refrigeration system disclosed for stadiums or arenas in Example 12 of the Sherwood '549 patent is commercially undesirable because it requires a large reservoir of secondary refrigerant. Such refrigerant, in its preferred form, is relatively expensive, and such a large reservoir (e.g., approximately 40 gallons) would make such a refrigeration system prohibitively expensive.
There thus remains the need for a refrigeration system that is suitable for providing the "blast chilling" effect needed for rapid chilling of massive quantities of product, such as beer or soda, for stadium event use, but that is economical and efficient in operation. Further, such a system would preferably use a secondary loop refrigeration agent which is nontoxic, nonflammable, environmentally friendly and does not require the use of high pressures. These and other needs are provided by the present invention, as disclosed herein.